Methods and apparatus by means of which tubes in a multi-tube heat transfer device may be cleaned of fouling deposits whilst the unit is maintained in operation are well known in the art. Apart from chemical cleaning methods, mechanical cleaning systems are typified by those made by Taprogge and Water Services of America, the use of which is almost totally confined to the cleaning of condenser tube bundles. In the Taprogge System, balls made of a soft, resilient material and with a diameter slightly larger than that of the tubes to be cleaned are released into the incoming cooling water flow of a condenser and are carried by the water through the tubes where they wipe the inner surfaces. Each tube is expected to receive a ball approximately every five minutes. Abrasive balls may be used initially for the removal of an existing consolidated fouling deposit. The balls are recovered from the outgoing water flow, being automatically gauged and replaced as wear occurs. The shortcomings of the Taprogge System are that the cleaning balls wear quite rapidly and, notwithstanding the installation of deflectors in the header box of the condenser, tend to pass more frequently through some tubes than others. The cleaning effect is thus not evenly distributed throughout the whole tube array. The Taprogge System has not been adapted to applications where elevated temperatures are involved and, for obvious reasons, it is not effective in the removal of deposits of contaminant material which have built up to more than a very slight degree. The cost of installing the Taprogge System is high and its use is therefore usually confined to applications where a specific and acute fouling problem exists.
The "Superscrubber System" made by Water Services of America uses brushing units which are captured in cages at the upstream ends of each tube. The brushing units are carried through the tubes by water flow and lodge in cages at their downstream ends. In a simple arrangement, the direction of cooling water flow through the condenser is reversed to repeat the cleaning process and return the brushing units to their original cages. Means have been devised to obviate the need to reverse the cooling water flow. These embody a radial duct which may be swept across the downstream ends of tubes to apply a localised water flow which redeploys the bushing units to the upstream ends of the tubes. U.S. Pat. Nos. 4,124,065, 4,269,264 and 4,353,414 describe various developments of this system. The shortcomings of the Water Services of America system are generally similar to those of the Taprogge System: it is expensive to install; it cannot remove existing consolidated fouling deposits; the brushing units wear and cannot be replaced whilst a heat transfer unit is on-line; brushes lodge in tubes and cannot be freed whilst the heat transfer unit is on-line; and its use is confined to applications which do not involve elevated temperatures.
Australian Patent Nos. 571,845, 572,181 and Patent Application No. 12079/88 disclosed a method of removing deposits of contaminant material from the inner surfaces of tubes by propelling a suitably dimensioned projectile through each tube by means of a virtually instantaneous release of a pressurised liquid. The diameter of the said projectile was less than that of the inner diameter of the tube to be cleaned and the cleaning mechanism was assumed to result from effects generated in the flow of liquid cleaning medium by the passage of the projectile. In the preferred embodiment, the projectile was simple, unitary and not fitted with scraping, abrading or brushing elements.